Laser-Based Measurements for Time and Frequency Domain Applications

High-resolution spectroscopic frequency measurements 393

5.12.6 Calcium standard

Other suitable candidates are represented by Ca and Mg. Indeed, the intercombination

transitions of alkaline earth a toms, like those shown in Figure 5.42, are celebrated examples

of references for optical frequency standar ds. Just to give an idea, Mg and Ca exhibit

natural linewidths as low as 0 .04 kHz and 0.4 kHz, res pectively. Additionally, in bo th cases,

the frequencies of the ∆m

J

= 0 transitions are almost insensitive to electric a nd magnetic

fields.

So far, most attention has focused on Ca atomic beams. Early work started in 1979 when

Barger et al. obtained a